Shielded cables having multiple conductors are used in a variety of environments, such as in housing structures or manufacturing facilities. The conductors in the cable are typically twisted together to form a bundle, and are covered along their length by an armor shield. Insulation is disposed around each of the conductors, and an additional layer of insulation can be disposed between the bundle of conductors and the armor shield to prevent faults from occurring between conductors or between the conductors and the shield.
The impact resistance of the shielded cable determines the suitability of a cable for a particular environment. The layers of insulation, along with the armor shield, provide the impact resistance for the cable. A less resistant cable can be used in milder environments, while a more resistant cable is necessary in harsher environments.
A variety of tests have been developed to test the impact resistance of a cable. The tests generally provide for impacting a selected portion of the cable with a test block of a particular weight from a selected height. The ability of the cable to withstand the impact of the block determines the impact resistance of the cable, and hence the suitability of the cable for a particular environment.
As it is being tested, the cable either withstands the impact of the test block, or a fault occurs within the cable. The fault can be caused by either a conductor being driven through the conductor insulation into contact with another conductor, or by a portion of the armor shield being driven through the outer layer of insulation into contact with a conductor. In either case, the impact resistance of the cable has been exceeded.
In order to determine if a fault has occurred in the cable during an impact test, a number of different types of fault detection circuits have been developed. For example, UL test #1569 tests the impact resistance of an eleven foot cable by impacting the cable at ten separate locations, wherein each location is at least twelve inches apart. Two of the insulated conductors in the cable are connected in series through a three watt, 120 volt neon lamp to the energized conductor of a two wire, grounded 120 volt 48-62 Hertz AC power supply. A third, bare or insulated grounding conductor in the cable is connected to the armor, to all parts of the impact apparatus, to earth ground, and to the grounded supply wire. The test block is impacted along the cable and the neon lamp indicates if a fault exists.
Additionally, Coleman et al., U.S. Pat. No. 3,736,503, discloses an electrical continuity and short circuit testing device for checking pairs of conductors in an insulated cable. One conductor in each pair is connected to the positive voltage supply from a step-up transformer, while a second conductor from each pair is connected to the negative voltage supply from the transformer. If a fault is present between the pair of conductors, current flows through the fault and a lamp is illuminated. Coleman further discloses providing a DC bias to illuminate the lamp after the fault is corrected.
Further, Le Doux, U.S. Pat. No. 3,377,551, discloses a multi-phase fault indicator circuit that utilizes current transformers to test conductors connected in-line to a load. The current transformers are formed by the test conductors as the primary windings, and the indicator circuit conductors as the secondary windings. The secondary windings each have two ends, wherein each end is wound around two phases of the primary windings at points between which faults are to be indicated. If a fault exists in the conductors, either as a phase-to-phase or a phase-to-ground fault, current is induced in the secondary windings and a voltage drop appears across a fault indicator to activate a protective relay.
The foregoing methods of testing attempt to provide fault indicator circuits which test conductor-to-conductor and conductor-to-ground faults in multiple conductor cables. However, these devices are not without drawbacks. For instance, although the UL specification alleges that the UL circuit is designed to test conductor-to-ground and conductor-to-conductor faults, the insulated conductors in the UL test are connected in-phase at the same potential. Accordingly, it has been applicant's experience that a potential difference will thereby only develop between a conductor shorted to ground, and not between two conductors shorted together. The UL test therefore only effectively measures conductor-to-ground faults and not conductor-to-conductor faults.
Moreover, the UL test and Coleman disclose high voltages and large currents flowing through a pair of shorted conductors. Additionally, Le Doux discloses test conductors connected in-line to a generator. Accordingly, the UL test, Coleman and Le Doux can thereby increase the risk of electrical shock to the operator during an impact test.